Quote:
Originally Posted by PRejto
Matthew,
I posed the question due to something I read recently. (names removed!)
"If I am using a Crayford style focuser using the typical friction drive configuration both on the cross shaft and the ball bearing reduction mechanism, which is typical, can and does the focusing program make adjustments based on absolute values, reported by step count, or if there is slip in the mechanism does it constantly make adjustments to negate the slippage.
Suppose you generate your V curves in the normal manner characterizing your system and then you go to do a focus but for a number of reasons there might be slippage between the ball bearing reduction drive and or the cross shaft and the friction plate on your Crayford Style focuser. Now since the actual position of the draw tube and the numbers displayed will not be the same does the focus program constantly adjust for this or is the effect cumulative?"
Answer(s):
"If you are relying on the motor steps to position your draw tube then you are out of luck - slippage may/may not be linear and it may/may not be predictable depending on load. BUT, you may have more success if you are monitoring the actual draw tube position with an external measuring system such as a precision digital .0001" resolution dial indicator. You would then have the challenge of developing an ASCOM compliant driver that would move the focuser until the desired reading is achieved. The best solution is to engineer the focuser so that slippage is not a concern with a given payload."
and
Interesting in reading your question regarding slippage on a Crayford focuser. We were plagued with that problem on our remote telescope in Spain. The focus would change a small amount depending on where the telescope was pointing. Was worse near the zenith as expected with gravity acting down helped by the weight of the imaging kit!
The solution was to change the Crayford for a FeatherTouch which uses a
rack and pinion focuser. That driven by a Lakeside (or a Robofocus)
motor focus works fine. No slippage with that. They're damned expensive
though, but its quality engineering you're getting for your bucks ;-)"
You can see why now I asked the question!
Peter
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Televues solution is exactly this. An external measurement of focuser position to make a closed loop. I opted for the moonlite with stepper and have had no issues with slippage.
https://lh3.googleusercontent.com/pr...G=w502-h293-nc